The heat treatment of metals and metal alloys is a very old and important industrial processing techology which is necessary for producing a very wide range of useful products. In particular, a large segment of this technology is related to surface and/or through hardening techniques which result in hard, wear and fatigue resistant surfaces superimposed on (in the case of surface hardening) or throughout (in the case of through hardening) a tough but relatively soft plain or alloy steel object. Common methods of currently achieving these results involve the use of either shell hardening, chemical flame hardening, or the use of induction hardening. See for example "Principle of Heat Treatment" by Ceorges KRAUS--ASM--1980--Chapter 10--or "Practical Heat Treating" by Howard E. Boyer--ASM--1984--Chapter 11.
Shell hardening is typically used in order to harden only the surface layers of a particular metal or metal alloy object. The specific object is usually immersed in a heating medium such as molten lead or a molten salt bath. This step is followed by quenching which produces a hard outer layer. This method of hardening is limited largely by part design (i.e. the entire part must be heated) and inherent limitations in the heating bath temperatures. These limitations prevent extremely rapid and localized heating in specific zones or areas of an object to be heat treated.
Flame hardening involves the use of a flame produced by a chemical reaction between an appropriate fuel gas and oxygen. Commonly used fuel gases are hydrogen or acetylene. Using this technique, objects to be selectively hardened are heated in the flame for an appropriate period of time, then rapidly quenched. The main limitation of this technique is that the chemical flame temperature limits the rate of heat transfer to the metal object and thus the rate of sample temperature increase is limited. This effect tends to allow conduction to overheat some specimen sections which are adjacent to the heat treated zone.
When the induction process is employed, the unhardened metal or metal alloy specimen is indirectly heated due to currents induced in the specimen by an external electromagnetic induction coil. There are a number of significant disadvantages associated with this technology. For example, localized temperature control is difficult to achieve using the induction hardening technique, especially on parts with sharp edges or varying thicknesses.
Since induction heating depends on currents induced within specific metallic specimens, some objects, (due to their geometrical shapes) cannot be hardened effectively using this technique. In addition, different shaped or sized objects each require their own specially designed induction coils so that the induction hardening process will be optimized. In some instances, the costs associated with the design and fabrication of these special induction coils can be prohibitive, especially if the objects which must be hardened are not manufactured in large quantities.